The present invention relates to a transmission system, and more particularly to a power-transmission arrangement which couples an electrical power unit to a step-down speed-reducing unit.
In the prior art, power units, such as electromotors, are provided with a conventional gear wheel transmission which reduces the relatively high rotational speeds of the electromotor to a desired lower value.
The material constants of the iron core laminations and of the winding, as well as the size of the air-gap formed between the stator and the rotor, impose limitations on the upper value of the magnitude of electrical energy which can be converted in a motor of a given volume, i.e. the energy density which can be converted is limited. The amount of electrical power which can be converted using a given volume, therefore is directly proportional to the rotational speed of the motor; for example, if a two-pole synchronous motor is connected to a 60 Hz, three-phase AC power supply, it will revolve at a speed of 3,600 rpm. This is the highest possible speed for direct connection to the 60 Hz power supply. The overall size of a four-pole motor, for example, which would revolve at 1,800 rpm, would have to be considerably increased for the same hp-delivery.
Since the energy density for the power transmitted between the teeth of a pair of transmission gear wheels is a multiple higher than that of the electromechanical conversion factor, the combination of a motor having a relatively high speed coupled with a gear-wheel transmission generally leads to the smallest-sized and most economical transmission arrangement of small and/or medium-sized drives with output speeds of less than 450 rpm.
In heavy-duty operation, noise reduction is an important consideration. A motor having two poles, i.e. having high rotational speed, requires a relatively high expenditure of money, labor and materials in order to dampen the resulting air noise and the noise resulting from magnetic forces in the motor. A gear-wheel transmission which is directly connected to a high-speed motor likewise requires to be noise-dampened to meet given noise dampening requirements. Nevertheless, for low output speed it is considerably cheaper and requires less volume to use a high speed motor with speed reduction gears rather than to use a motor of high number of poles, which produces the low speed without additional gears. It is particularly advantageous to have a small number of gear wheels in order to reduce the size, the noise and the price of the transmission system.
In order to optimize the magnitude of the electrical power to be efficiently converted and the desired low speed of rotation output with such considerations as volume, noise, weight and price, various trade-offs must be made amongst the following factors:
1. Heavy-duty operation and high speed of the power unit are desired in order to maximize the efficiency of operation.
2. In order to save space, the increased air noise and higher magnetic noise of motors having high energy density, must be accepted or else, increased expenditures for damping such noises are necessary.
3. A multi-staged gear-transmission system means that each additional stage increases the cost, worsens the efficiency and increases the volume since the energy must be converted in full at each stage.
In the prior art, the general theory of operation of a harmonic drive gear is known. See, for example, German patent PS 1,135,259, Issued Mar. 7, 1963 wherein an elliptical element deforms a steel sleeve having external teeth gearing. A steel ring has internal teeth gearing which meshes with the external teeth. The number of internal teeth on the steel ring is more than the number provided on the sleeve so that speed-reduction can be realized. Moreover, since the sleeve has fewer teeth, its diameter is correspondingly smaller than the diameter of the surrounding steel ring so that the sleeve can fit within the ring, and both sets of teeth can mesh. Typically, the region of meshing engagement is such that, at any one particular moment for the preferred diameters if the sleeve and ring, the number of teeth in mutual engagement with each other is about 15% of the entire number of teeth. The elliptical element imparts its curvature to the sleeve.
In the prior art, whenever it is desired to connect a harmonic drive gear, i.e. the elliptical element, to an electrical power unit, the harmonic drive gear is always connected to the output end of the shaft of the power unit; in other words, the gear is located externally of the power unit housing. This increases the axial distance between the harmonic gear and the electrical power unit and results in a relatively large-sized transmission system which, in turn, results in the aforementioned disadvantages.